According to several reports UK registrar and hosting company UK Reg has been experiencing intermittent issues.
More details are available here and here
The company issued a statement to explain what happened:
123-reg experienced intermittent performance issues on its DNS servers between late afternoon on Friday 16 November and Sunday 18 November. This meant that some customers have encountered difficulties with their domain names during this period.
This problem was caused by a combination of excessive loading on the DNS servers and a rare hardware failure. During this time, 123-reg engineers have replaced the hardware and full service has been resumed.
We apologise to our customers for the inconvenience that the outage would have caused and we have begun an investigation to identify the cause of the failure, and any necessary actions required will be implemented without delay. Further information and updates is available from http://www.hosting-status.pipex.net